Ordinarily the core material detonates but in some types rapid deflagration or pyrotechnic reaction suffices as when the tubing is connected to a detonator within which a deflagration to detonation transition occurs. The signal transmission tubing is itself initiated by an electric cap, a non-electric detonator, an electric discharge device or indeed by any other means capable of initiating the required self-sustaining reaction or detonation of the core material. A favoured type of low energy fuse is the so-called shock tube as described in, and cross-referenced in, European Patent No. 327 219 (ICI).
This invention relates particularly to shock tube fuses. For present purposes, a shock tube fuse is one in which an initiation signal for a non-electric signal delay device or detonator (instantaneous or delay) is transmitted through an unobstructed internal bore of an extruded flexible plastics tubing by induced detonation of a contained unconsolidated mixture of particles of reacting substances loosely adherent to the bore surfaces and distributed thereover as a shock-dislodgeable dusting. The plastics material of which the tubing is formed may suitably be as described in the prior art referenced hereinbefore. The internal bore of the tubing is usually narrow; and is usually circular (though it need not be) o Common shock tube fuse dimensions are I.D. 1.3 mm, O.D. 3.0 mm, but the trend is towards smaller bores, less plastics usage, and lower mass per unit length of reaction mixture. For most practical purposes the bore volume per metre of length will be less than .pi./2.times.10.sup.-6 m.sup.3, and may be less than .pi./4.times.10.sup.-6 m.sup.3, corresponding to I.Ds. of circular cross-section tubing of about 1.4 and 1.0 mm respectively.
The core loading of reacting substances in shock tube fuses in use today is commonly in the range of from 15 to 30 mg/m of tube length (where the tube has an I.D. of around 1.3 mm) or 8 to 20 mg/m where the tube has a smaller I.D. say under 1 mm. These figures correspond to a loading per square metre of tube inner surface of below 10 g, and to a loading per cubic metre of tube bore volume of about 10-30.times.10.sup.3 g. These figures for surface area loading and bore volume loading are better guidelines for choosing suitable tube loadings in mg/m of tube than the above quoted mg/m figures where the inner bore of the plastics tube is other than circular in cross-section.
A preferred method of producing a shock tube fuse is to extrude a suitable plastics material capable of forming, on cooling, a permanent chosen tubular form and possessing requisite inner surface affinity for particulate reacting mixture, and simultaneously through the extrusion head introducing the particulate reacting mixture in to the interior of the tube whereupon it becomes loosely adherent, but shock-dislodgeable, on the inner tube bore surface. A presently favoured reacting mixture is a mixture of aluminium and HMX in a 6:94 weight ratio. However, this mixture (as in HMX alone) is quite sensitive to the levels of temperature which need to be developed for rapid extrusion of tube-forming plastics and a graph of "time to reaction" vs sample temperature for these substances quantifies the risk of runaway reaction with all the attendant hazards. The test which enables this graph to be drawn is the Henkin McGill Test, described in the literature. This thermal sensitivity imposes constraints on the tube extrusion technology, on the choice of plastics, and on the rate of tube extrusion having regard to the effectiveness of the cooling system used to bring about tube consolidation at the chosen cross-sectional I.D./O.D.